Why can't I UV Unwrap this model like this? Extreme stretching
The problem lies in the ngons so you have two options here.

a: Select each faces you want to have an island of and unwrap/smart unwrap it. This won't attach both side as one island but it will give you consistent uv maps for both sides. The UV islands will be overlapping so you want to pack the island.


b: get rid of the ngons: 
it looks a bit botched because I knife cut the faces in a hurry.

edit:
You might want to change the method and check/uncheck fill holes to your liking